Reproductive and developmental toxicity of dioxin in fish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD or dioxin) is a global environmental contaminant and the prototypical ligand for investigating aryl hydrocarbon receptor (AHR)-mediated toxicity. Environmental exposure to TCDD results in developmental and reproductive toxicity in fish, birds and mammals. To resolve the ecotoxicological relevance and human health risks posed by exposure to dioxin-like AHR agonists, a vertebrate model is needed that allows for toxicity studies at various levels of biological organization, assesses adverse reproductive and developmental effects and establishes appropriate integrative correlations between different levels of effects. Here we describe the reproductive and developmental toxicity of TCDD in feral fish species and summarize how using the zebrafish model to investigate TCDD toxicity has enabled us to characterize the AHR signaling in fish and to better understand how dioxin-like chemicals induce toxicity. We propose that such studies can be used to predict the risks that AHR ligands pose to feral fish populations and provide a platform for integrating risk assessments for both ecologically relevant organisms and humans.

Sensitivity to dioxin decreases as zebrafish mature

The embryos of teleost fish are exquisitely sensitive to the toxic effects of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, several lines of evidence suggest that adults are less sensitive to TCDD. To better understand and characterize this difference between early life stage and adults, we exposed zebrafish (Danio rerio) to graded TCDD concentrations at different ages. The LD(50) for embryos exposed at 1 day post-fertilization (dpf) was more than an order of magnitude lower than it was for juveniles exposed at 30 dpf. The latency between exposure and response also increased with age. Embryo toxicity was characterized by marked cardiovascular collapse and heart malformation, whereas juveniles exposed at 30 dpf had no detectable cardiovascular toxicity. In juveniles, the effects of TCDD exposure included stunted growth, altered pigmentation, and skeletal malformations. Furthermore, the transcriptional profile produced in hearts exposed to TCDD as embryos had very little overlap with the transcriptional changes induced by TCDD at 30 dpf. The early cardiotoxic response was associated with fish exposed prior to metamorphosis from the larval to the adult body plan at approximately 14 dpf. Our results show conclusively that the developmental stage at the time of exposure controls the toxic response to TCDD.

A dominant negative zebrafish Ahr2 partially protects developing zebrafish from dioxin toxicity

The toxicity by 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) is thought to be caused by activation of the aryl hydrocarbon receptor (AHR). However, our understanding of how AHR activation by TCDD leads to toxic effects is poor. Ideally we would like to manipulate AHR activity in specific tissues and at specific times. One route to this is expressing dominant negative AHRs (dnAHRs). This work describes the construction and characterization of dominant negative forms of the zebrafish Ahr2 in which the C-terminal transactivation domain was either removed, or replaced with the inhibitory domain from the Drosophila engrailed repressor protein. One of these dnAhr2s was selected for expression from the ubiquitously active e2fα promoter in transgenic zebrafish. We found that these transgenic zebrafish expressing dnAhr2 had reduced TCDD induction of the Ahr2 target gene cyp1a, as measured by 7-ethoxyresorufin-O-deethylase activity. Furthermore, the cardiotoxicity produced by TCDD, pericardial edema, heart malformation, and reduced blood flow, were all mitigated in the zebrafish expressing the dnAhr2. These results provide in vivo proof-of-principle results demonstrating the effectiveness of dnAHRs in manipulating AHR activity in vivo, and demonstrating that this approach can be a means for blocking TCDD toxicity.

Molecular targets that link dioxin exposure to toxicity phenotypes

Many toxicology studies have elucidated health effects associated with exposure to various chemicals, but few have identified the molecular targets that cause specific endpoints of toxicity. Our understanding of the toxicity of dioxins, a group of chemicals capable of causing toxicity at environmentally relevant levels of exposure, is no exception. Dioxins are unique compared to most chemicals that we are exposed to in the environment because they activate a high affinity receptor, aryl hydrocarbon receptor (AhR), that was identified more than three decades ago. In recent years, several lines of experimental evidence have provided clues for opening the "black box" that contains the molecular mechanisms of dioxin action. These clues have emerged by toxicologists beginning to identify the molecular targets that link AhR signaling to tissue-specific toxicity phenotypes. Endpoints of dioxin toxicity for which downstream molecular targets have begun to be elucidated are observed in developmental or tissue regeneration processes, and include impaired prostate development and hydronephrosis in mouse fetuses and pups, reduced midbrain blood flow and jaw malformation in zebrafish embryos, and impaired fin regeneration in larval and adult zebrafish. Significant progress in identifying molecular targets for dioxin-induced hepatotoxicity in adult mice also has occurred. Misregulation of AhR downstream pathways, such as conversion of arachidonic acid to prostanoids via cyclooxygenase-2, and altered Wnt/β-catenin signaling downregulating Sox9, and signaling by receptors for inflammatory cytokines have been implicated in tissue-specific endpoints of dioxin toxicity. These findings may not only begin to clarify the molecular targets of dioxin action but shed light on new molecular events associated with development and disease.

Latent TGF-β binding protein 3 identifies a second heart field in zebrafish

The four-chambered mammalian heart develops from two fields of cardiac progenitor cells (CPCs) distinguished by their spatiotemporal patterns of differentiation and contributions to the definitive heart [1–3]. The first heart field differentiates earlier in lateral plate mesoderm, generates the linear heart tube and ultimately gives rise to the left ventricle. The second heart field (SHF) differentiates later in pharyngeal mesoderm, elongates the heart tube, and gives rise to the outflow tract (OFT) and much of the right ventricle. Because hearts in lower vertebrates contain a rudimentary OFT but not a right ventricle [4], the existence and function of SHF-like cells in these species has remained a topic of speculation [4–10]. Here we provide direct evidence from Cre/Lox-mediated lineage tracing and loss of function studies in zebrafish, a lower vertebrate with a single ventricle, that latent-TGFβ binding protein 3 (ltbp3) transcripts mark a field of CPCs with defining characteristics of the anterior SHF in mammals. Specifically, ltbp3⁺ cells differentiate in pharyngeal mesoderm after formation of the heart tube, elongate the heart tube at the outflow pole, and give rise to three cardiovascular lineages in the OFT and myocardium in the distal ventricle. In addition to expressing Ltbp3, a protein that regulates the bioavailability of TGFβ ligands [11], zebrafish SHF cells co-express nkx2.5, an evolutionarily conserved marker of CPCs in both fields [4]. Embryos devoid of ltbp3 lack the same cardiac structures derived from ltbp3⁺ cells due to compromised progenitor proliferation. Additionally, small-molecule inhibition of TGFβ signaling phenocopies the ltbp3-morphant phenotype whereas expression of a constitutively active TGFβ type I receptor rescues it. Taken together, our findings uncover a requirement for ltbp3-TGFβ signaling during zebrafish SHF development, a process that serves to enlarge the single ventricular chamber in this species.

Role of zebrafish cytochrome P450 CYP1C genes in the reduced mesencephalic vein blood flow caused by activation of AHR2

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes various signs of toxicity in early life stages of vertebrates through activation of the aryl hydrocarbon receptor (AHR). We previously reported a sensitive and useful endpoint of TCDD developmental toxicity in zebrafish, namely a decrease in blood flow in the dorsal midbrain, but downstream genes involved in the effect are not known. The present study addressed the role of zebrafish cytochrome P450 1C (CYP1C) genes in association with a decrease in mesencephalic vein (MsV) blood flow. The CYP1C subfamily was recently discovered in fish and includes the paralogues CYP1C1 and CYP1C2, both of which are induced via AHR2 in zebrafish embryos. We used morpholino antisense oligonucleotides (MO or morpholino) to block initiation of translation of the target genes. TCDD-induced mRNA expression of CYP1Cs and a decrease in MsV blood flow were both blocked by gene knockdown of AHR2. Gene knockdown of CYP1C1 by two different morpholinos and CYP1C2 by two different morpholinos, but not by their 5 nucleotide-mismatch controls, was effective in blocking reduced MsV blood flow caused by TCDD. The same CYP1C-MOs prevented reduction of blood flow in the MsV caused by β-naphthoflavone (BNF), representing another class of AHR agonists. Whole-mount in situ hybridization revealed that mRNA expression of CYP1C1 and CYP1C2 was induced by TCDD most strongly in branchiogenic primordia and pectoral fin buds. In situ hybridization using head transverse sections showed that TCDD increased the expression of both CYP1Cs in endothelial cells of blood vessels, including the MsV. These results indicate a potential role of CYP1C1 and CYP1C2 in the local circulation failure induced by AHR2 activation in the dorsal midbrain of the zebrafish embryo.

Characterization of the adult zebrafish cardiac proteome using online pH gradient strong cation exchange-RP 2D LC coupled with ESI MS/MS

2D HPLC separations by coupling strong cation exchange (SCX) and RP fractionation have been widely used in large-scale proteomic studies. Traditionally this method is performed by salt gradient SCX separation followed by RP and MS/MS analysis. The salt gradient SCX method has been known to have low peptide and protein resolution. In this study, we implemented a pH gradient SCX-RP HPLC platform to separate proteome digests from adult zebrafish hearts, followed by ESI quadrupole-TOF MS/MS analysis. This pH gradient SCX method has improved peptide separation, as demonstrated by a greater number of peptides and proteins identified from individual SCX fractions. This pH gradient method also has better MS compatibility owing to lower salt usage. This setup allows fast microflow fractionation in SCX dimension and nanoflow RP separation in the second dimension, and can be easily implemented on conventional capillary LC ESI MS/MS systems. Using this setup, we identified 1375 proteins from adult zebrafish hearts, establishing the first reported experimental data set for the heart proteome of zebrafish. This work laid the foundation for further studies of environmental cardiac toxicology using zebrafish as a model organism.

Malformation of certain brain blood vessels caused by TCDD activation of Ahr2/Arnt1 signaling in developing zebrafish

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes various signs of toxicity in early life stages of vertebrates through activation of the aryl hydrocarbon receptor (AHR). The AHR also plays important roles in normal development in mice, and AHR(-/-) mice show abnormal development of vascular structures in various blood vessels. Our previous studies revealed that Ahr type 2 (Ahr2) activation by TCDD and beta-naphthoflavone (BNF) caused a significant decrease in blood flow in the dorsal midbrain of zebrafish embryos. Here we report effects of TCDD exposure on the morphology of some blood vessels in the head of developing zebrafish. TCDD caused concentration-dependent anatomical rearrangements in the shape of the prosencephalic artery in zebrafish larvae. In contrast, no major vascular defects were recognized in the trunk and tail regions following exposure to TCDD at least at the concentrations used. Essentially, the same observations were also confirmed in BNF-exposed larvae. Knock-down of either Ahr2 or Ahr nuclear translocator type 1 (Arnt1) by morpholino oligonucleotides (MOs) protected larvae against abnormal shape of the prosencephalic artery caused by TCDD and BNF. On the other hand, knock-down of Ahr2 or Arnt1 in vehicle-exposed zebrafish larvae had no clear effect on morphology of the prosencephalic artery or trunk vessels. Ascorbic acid, an antioxidant, protected against the TCDD-induced decrease in blood flow through the prosencephalic artery, but not the abnormal morphological changes in the shape of this artery. These results indicate that activation of Ahr2/Arnt1 pathway by TCDD and BNF affects the shape of certain blood vessels in the brain of developing zebrafish.

Microbial detoxification of aflatoxin

Yeasts, molds, bacteria, actinomycetes, algae, and fungal spores were screened for their ability to degrade aflatoxin. Some molds and mold spores partially transformed aflatoxin B(1) to new fluorescing compounds. Only one of the bacteria, Flavobacterium (aurantiacum?) NRRL B-184, removed aflatoxin from solution. Both growing and resting cells of B-184 took up toxin irreversibly. Toxin-contaminated milk, oil, peanut butter, peanuts, and corn were completely detoxified, and contaminated soybean was partially detoxified by addition of B-184. Duckling assays showed that detoxification of aflatoxin solutions by B-184 was complete, with no new toxic products being formed.

Androgenic regulation of ventral epithelial bud number and pattern in mouse urogenital sinus

The ventral urogenital sinus (UGS) of control male mice has two rows of 3-4 prostatic buds at birth, but how androgens regulate ventral bud (VB) number and patterning is unclear. VBs in both sexes appeared to be a mixture of prostatic and urethral buds. UGSs from Tfm male and antiandrogen (flutamide)-exposed mice had small VBs, suggesting that initiation of some VBs is androgen independent. Tfm male mice are widely considered completely androgen insensitive yet their UGSs were 5alpha-dihydrotestosterone (DHT)- responsive. VBs (6-8) were generally distributed bimodally on the left-right axis at both minimal and normal male androgen signaling. Yet control females and DHT-exposed Tfm males had 13-14 VBs, whose left-right distribution was fairly uniform. These results suggest that VB number and distribution respond biphasically as androgen signaling increases from minimal, and that androgens regulate bud specification. Complete VB agenesis by the selective budding inhibitor 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) required high androgen signaling.

2,3,7,8-Tetrachlorodibenzo-p-dioxin inhibits fibroblast growth factor 10-induced prostatic bud formation in mouse urogenital sinus

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) dorsalizes the pattern of prostatic buds developing from the urogenital sinus (UGS) of male fetal mice, causing some buds to form in inappropriate positions while blocking formation of others. This teratogenic TCDD action significantly reduces prostate main duct number and causes ventral prostate agenesis in exposed males. The purpose of this study was to determine whether inhibition of fibroblast growth factor 10 (FGF10) signaling is mechanistically linked to mouse prostatic budding impairment by TCDD. In utero TCDD exposure induced aryl hydrocarbon receptor-responsive cytochrome P450 1b1 messenger RNA (mRNA) in ventral UGS regions where Fgf10 and fibroblast growth factor receptor 2 (Fgfr2) mRNA were expressed and where budding was most severely inhibited by TCDD. However, TCDD exposure did not reduce Fgf10 or Fgfr2 mRNA abundance in the UGS or alter their distribution. Addition of FGF10 protein to UGS organ culture media increased the abundance of UGS basal epithelial cells immunopositive for phosphorylated extracellular signal-regulated kinase (ERK). FGF10 also increased the number of 5-bromo-2'-deoxyuridine (BrdU)-labeled UGS epithelial cells and increased the number of prostatic buds formed per UGS. Addition of TCDD to UGS organ culture media did not alter FGF10-induced ERK activation in UGS basal epithelium but prevented FGF10-induced BrdU incorporation and blocked FGF10-induced prostatic bud formation. These results identify basal urogenital sinus epithelium cells as the key site of FGF10 action during fetal prostate development and suggest that TCDD likely acts downstream of FGFR2 and ERK to restrict UGS epithelial cell proliferation and prevent prostatic bud formation.

Histone deacetylases facilitate sodium/calcium exchanger up-regulation in adult cardiomyocytes

It is becoming increasingly evident that histone deacetylases (HDACs) have a prominent role in the alteration of gene expression during the growth remodeling process of cardiac hypertrophy. HDACs are generally viewed as corepressors of gene expression. However, we demonstrate that class I and class II HDACs play an important role in the basal expression and up-regulation of the sodium calcium exchanger (Ncx1) gene in adult cardiomyocytes. Treatment with the HDAC inhibitor trichostatin A (TSA) prevented the pressure-overload-stimulated up-regulation of Ncx1 expression. Overexpression of HDAC5 resulted in the dose-dependent up-regulation of basal and alpha-adrenergic stimulated Ncx1 expression. We show that Nkx2.5 recruits HDAC5 to the Ncx1 promoter, where HDAC5 complexes with HDAC1. Nkx2.5 also interacts with transcriptional activator p300, which is recruited to the Ncx1 promoter. We demonstrate that when Nkx2.5 is acetylated, it is found associated with HDAC5, whereas deacetylated Nkx2.5 is in complex with p300. Notably, TSA treatment prevents p300 from being recruited to the endogenous Ncx1 promoter, resulting in the repression of Ncx1 expression. We propose a novel model for Ncx1 regulation in which deacetylation of Nkx2.5 is required for the recruitment of p300 and results in up-regulation of exchanger expression.

Potential roles of Arnt2 in zebrafish larval development

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a basic helix-loop-helix-PAS heterodimeric transcription factor that dimerizes with other basic helix-loop-helix-PAS proteins to mediate biological responses. The function of ARNT2 is poorly understood. Here we provide an initial characterization of the zebrafish arnt2 null (arnt2(-/-)) mutant to identify functions of Arnt2 during development. Arnt2(-/-) mutant zebrafish develop normally until 120 hours postfertilization (hpf ) when morphological changes and functional deficits occur. The C-start escape response initiated by either touch or startle stimuli is absent in the mutants. Brain ventricle size is markedly increased at 120 hpf. Heart ventricles are enlarged, with decreased ventricle wall thickness. A cardiac arrhythmia, characterized by missing beats, is also observed in the mutants. This is associated with bradycardia in arnt2(-/-) larvae. Dilated liver sinusoids merge abnormally to form an extensive, labyrinth-like network of vascular channels. External appearance of arnt2(-/-) larvae at 120 hpf is indistinguishable from wild type except that the swim bladder is not inflated. The arnt2(-/-) mutants are not debilitated when phenotypic effects are first detected at 120 hpf that culminate in mortality, 4 days later around 216 hpf. Gross morphological assessment of the development of forebrain, midbrain, and hindbrain regions, neuromasts and Mauthner neurons, inner ear semicircular canals and otoliths, primary motor neurons, trigeminal ganglia, and trunk skeletal muscles, before or when the arnt2(-/-) phenotype was observed, failed to demonstrate a difference from wild type. The only effect in arnt2(-/-) larvae that occurred before 120 hpf was a decrease in expression of sim1, an Arnt2 dimerization partner, in the hypothalamus and ventral thalamus at 72 hpf. Further research is needed to determine if the primary functions of Arnt2 occur during the larval stage, when the phenotype is observed, or earlier in development.

Persistent adverse effects on health and reproduction caused by exposure of zebrafish to 2,3,7,8-tetrachlorodibenzo-p-dioxin during early development and gonad differentiation

Little is understood regarding the impacts of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure during early development on the health, survival, and reproductive capability of adults. Here we use zebrafish to determine whether early life stage exposure to TCDD induces toxicity in adult zebrafish and their offspring. Zebrafish were exposed to graded concentrations of TCDD (0-400 pg/ml) via waterborne exposure for 1 h/week from 0 to 7 weeks of age. The heart and swim bladder were identified as being most sensitive to TCDD exposure during early development. Subtle developmental toxic responses collectively impaired survival, and only zebrafish in the 0, 25, and 50 pg TCDD/ml groups survived to adulthood. Surviving fish exhibited TCDD toxicity in craniofacial structures (i.e., operculum and jaw), heart, swim bladder, and ovary. Exposure to 25 pg TCDD/ml impaired egg production (40% of control), fertility (90% of control), and gamete quality. TCDD-treated males contributed more than females to impaired reproductive capacity. Transgenerational effects were also discovered in that offspring from parents exposed to TCDD during early life stages showed a 25% increase in mortality compared with the F1 of dimethyl sulfoxide fish, reduced egg production (30-50% of control) and fertility (96% of control). Thus, adverse effects resulting from TCDD exposure during early life stages for one generation of zebrafish were sufficient to cause adverse health and reproductive effects on a second generation of zebrafish. In the environment, transgenerational effects such as these may contribute to population declines for the most TCDD sensitive fish species.

Quantum dot nanotoxicity assessment using the zebrafish embryo

Quantum dots (QDs) hold promise for several biomedical, life sciences, and photovoltaic applications. Substantial production volumes and environmental release are anticipated. QD toxicity may be intrinsic to their physicochemical properties, or result from the release of toxic components during breakdown. We hypothesized that developing zebrafish could be used to identify and distinguish these different types of toxicity. Embryos were exposed to aqueous suspensions of CdSe(core)/ZnS(shell) QDs functionalized with either poly-L-lysine or poly(ethylene glycol) terminated with methoxy, carboxylate, or amine groups. Toxicity was influenced by the QD coating, which also contributed to the QD suspension stability. At sublethal concentrations, many QD preparations produced characteristic signs of Cd toxicity that weakly correlated with metallothionein expression, indicating that QDs are only slightly degraded in vivo. QDs also produced distinctly different toxicity that could not be explained by Cd release. Using the zebrafish model, we were able to distinguish toxicity intrinsic to QDs from that caused by released metal ions. We conclude that developing zebrafish provide a rapid, low-cost approach for assessing structure-toxicity relationships of nanoparticles.

The selective aryl hydrocarbon receptor modulator 6-methyl-1,3,8-trichlorodibenzofuran inhibits prostate tumor metastasis in TRAMP mice

The aryl hydrocarbon receptor (AhR) is a basic-helix-loop-helix transcription factor that binds halogenated aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and endogenous compounds. We previously reported that AhR null (Ahr(-/-)) transgenic adenocarcinoma of the mouse prostate (TRAMP) mice on a C57BL/6J background develop prostate tumors with much greater frequency than AhR wild-type (Ahr(+/+)) TRAMP mice, suggesting that the AhR has tumor suppressor properties. Because AhR signaling pathway inactivation increased susceptibility to prostate tumorigenesis, we tested the hypothesis that a selective AhR modulator (SAhRM), 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), can protect against prostate tumorigenesis. TRAMP mice on the standard C57BL/6JxFVB genetic background were fed 0, 10, or 40mg 6-MCDF/kg diet beginning at 8 weeks of age. Tumor incidence, pelvic lymph node metastasis, and serum vascular endothelial growth factor (VEGF) concentrations were determined at 140 days of age. Prostate tumor incidence and size were not significantly reduced in mice fed 6-MCDF. However, the frequency of pelvic lymph node metastasis was reduced fivefold in mice fed the 40mg 6-MCDF/kg diet. Serum VEGF concentrations were also reduced by 6-MCDF treatment, particularly in mice without prostate tumors, and 6-MCDF was shown to act directly on cultured prostates to inhibit VEGF secretion. Together, these results suggest that 6-MCDF inhibits metastasis, in part, by inhibiting prostatic VEGF production prior to tumor formation. This is the first report that 6-MCDF can confer protection against prostate cancer in vivo.

AHR signaling in prostate growth, morphogenesis, and disease

Most evidence of aryl hydrocarbon receptor (AHR) signaling in prostate growth, morphogenesis, and disease stems from research using 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to pharmacologically activate the AHR at various stages of development. This review discusses effects of TCDD on prostate morphogenesis and highlights interactions between AHR and other signaling pathways during normal and aberrant prostate growth. Although AHR signaling modulates estrogen and androgen signaling in other tissues, crosstalk between these steroid hormone receptors and AHR signaling cannot account for actions of TCDD on prostate morphogenesis. Instead, the AHR appears to act within a cooperative framework of developmental signals to regulate timing and patterning of prostate growth. Inappropriate activation of AHR signaling as a result of early life TCDD exposure disrupts the balance of these signals, impairs prostate morphogenesis, and has an imprinting effect on the developing prostate that predisposes to prostate disease in adulthood. Mechanisms of AHR signaling in prostate growth and disease are only beginning to be unraveled and recent studies have revealed its interactions with WNT5A, retinoic acid, fibroblast growth factor 10, and vascular endothelial growth factor signaling pathways.

Differences in gene expression and benzo[a]pyrene-induced DNA adduct formation in the liver of three strains of female mice with identical AhRb2 genotype treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin and/or benzo[a]pyrene

To search for genes whose products modify aryl hydrocarbon receptor (AhR)-dependent toxicity caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), gene expression profiles in the liver were surveyed using microarrays 24 h after the administration of TCDD to three strains of female mice, BALB/cAnN (BALB), C3H/HeN (C3H) and CBA/JN (CBA) all of identical AhR genotype. The BALB/cAnN strain had a more marked induction of a number of glutathione S-transferase (GST) sub-families, particularly the GSTmicro gene family, compared with the other two strains. To assess the effects of GSTs induction to metabolize carcinogens, TCDD (40 microg kg(-1)) was administered to BALB and CBA strains, followed 24 h later by an i.p. injection of low or high dose of benzo[a]pyrene (B[a]P, 50 or 200 mg kg(-1)). The 32P-postlabelling analysis showed that administration of TCDD alone failed to induce DNA adduct formation in both BALB and CBA strain mouse livers. The low dose of B[a]P alone produced DNA adduct in the liver of both strains to a similar extent. Treatment with TCDD 24 h before the low dose of B[a]P suppressed the formation of B[a]P-induced DNA-adduct more markedly in the BALB strain compared with the CBA strain. Taken together, these findings show that TCDD treatment causes strain-specific alterations in gene expression and B[a]P-induced DNA adduct formation in the liver of female mice of the same AhRb2 genotype. Furthermore, it suggests that TCDD-treated female mice of the BALB strain may have genes whose products modify the toxicity of B[a]P as evidenced by TCDD-induced alterations in B[a]P-DNA adduct formation.

Retinoic acid induces prostatic bud formation

Formation of prostatic buds from the urogenital sinus (UGS) to initiate prostate development requires localized action of several morphogenetic factors. This report reveals all-trans-retinoic acid (RA) to be a powerful inducer of mouse prostatic budding that is associated with reciprocal changes in expression of two regulators of budding: sonic hedgehog (Shh) and bone morphogenetic protein 4 (Bmp4). Localization of retinoid signaling and expression of RA synthesis, metabolism, and receptor genes in the UGS on embryonic days 14.5-17.5 implicate RA in the mechanism of bud initiation. In UGS organ culture, RA increased prostatic budding, increased Shh expression, and decreased Bmp4. Prostatic budding was stimulated in the absence of RA by recombinant SHH, by blocking BMP4 signaling with NOGGIN, or by combined treatment with SHH and NOGGIN in UGS organ culture media. These observations suggest that reciprocal changes in hedgehog and BMP signaling by RA may regulate bud initiation.

Role of the cyclooxygenase 2-thromboxane pathway in 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced decrease in mesencephalic vein blood flow in the zebrafish embryo

Previously, we reported that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) evoked developmental toxicity required activation of aryl hydrocarbon receptor type 2 (AHR2), using zebrafish embryos. However, the downstream molecular targets of AHR2 activation are largely unknown and are the focus of the present investigation. TCDD induces cyclooxygenase 2 (COX2), a rate-limiting enzyme for prostaglandin synthesis in certain cells. In the present study, we investigated the role of the COX2-thromboxane pathway in causing a specific endpoint of TCDD developmental toxicity in the zebrafish embryo, namely, a decrease in regional blood flow in the dorsal midbrain. It was found that the TCDD-induced reduction in mesencephalic vein blood flow was markedly inhibited by selective COX2 inhibitors, NS-398 and SC-236, and by a general COX inhibitor, indomethacin, but not by a selective COX1 inhibitor, SC-560. Gene knock-down of COX2 by two different types of morpholino antisense oligonucleotides, but not by their negative homologs, also protected the zebrafish embryos from mesencephalic vein circulation failure caused by TCDD. This inhibitory effect of TCDD on regional blood flow in the dorsal midbrain was also blocked by selective antagonists of the thromboxane receptor (TP). Treatment of control zebrafish embryos with a TP agonist also caused a reduction in mesencephalic vein blood flow and it too was blocked by a TP antagonist, without any effect on trunk circulation. Finally, gene knock-down of thromboxane A synthase 1 (TBXS) with morpholinos but not by the morpholinos' negative homologs provided significant protection against TCDD-induced mesencephalic circulation failure. Taken together, these results point to a role of the prostanoid synthesis pathway via COX2-TBXS-TP in the local circulation failure induced by TCDD in the dorsal midbrain of the zebrafish embryo.

Aryl hydrocarbon receptor-mediated down-regulation of sox9b causes jaw malformation in zebrafish embryos

Exposure to environmental contaminants can disrupt normal development of the early vertebrate skeleton. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) impairs craniofacial skeletal development across many vertebrate species, and its effects are especially prominent in early life stages of fish. TCDD activates the aryl hydrocarbon receptor, a transcription factor that mediates most if not all TCDD responses. We investigated the transcriptional response in the developing zebrafish jaw after TCDD exposure using DNA microarrays. Zebrafish larvae were exposed to TCDD at 96 h after fertilization, and jaw cartilage tissue was harvested for microarray analysis at 1, 2, 4, and 12 h after exposure. Numerous chondrogenic transcripts were misregulated by TCDD in the jaw. Comparison of transcripts altered by TCDD in jaw with transcripts altered in embryonic heart showed that the transcriptional responses in the jaw and the heart were strikingly different. Sox9b, a critical chondrogenic transcription factor, was the most significantly reduced transcript in the jaw. We hypothesized that the TCDD reduction of sox9b expression plays an integral role in affecting the formation of the embryonic jaw. Morpholino knockdown of sox9b expression demonstrated that partial reduction of sox9b expression alone was sufficient to produce a TCDD-like jaw phenotype. Loss of a single copy of the sox9b gene in sox9b(+/-) heterozygotes increased sensitivity to jaw malformation by TCDD. Finally, embryos injected with sox9b mRNA and then exposed to TCDD blocked TCDD-induced jaw toxicity in approximately 14% of sox9b-injected embryos. These results suggest that reduced sox9b expression in TCDD-exposed zebrafish embryos contributes to jaw malformation.

Dioxin causes ventral prostate agenesis by disrupting dorsoventral patterning in developing mouse prostate

Prostate ductal development is initiated by androgen-dependent signals in fetal urogenital sinus (UGS) mesenchyme that stimulate prostatic bud formation in UGS epithelium. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD, 5 microg/kg maternal dose) inhibited ventral and dorsolateral but not anterior prostatic budding. We sought to determine which stage of budding, specification or initiation, was inhibited. Ventral prostatic bud formation was maximally inhibited when TCDD exposure spanned E15.5-16.5 and dorsolateral prostatic bud formation when it spanned E14.5-15.5. Because ventral and dorsolateral buds are specified at these times, TCDD impaired bud specification. We hypothesized that TCDD inhibited ventral bud specification by forming a continuous smooth muscle barrier between UGS mesenchyme and epithelium in the ventral prostatic UGS region, blocking mesenchymal-epithelial signaling, but no such barrier was found. We hypothesized that increased aryl hydrocarbon receptor (AHR) signaling in ventral and dorsolateral UGS increased their sensitivity to TCDD, but levels of AHR nuclear translocator (ARNT) protein, Ahr mRNA, and AHR-dependent gene expression were not higher than in anterior UGS where budding was unaffected. However, we identified overlapping expression of Ahr, ARNT, and AHR-induced transcripts in the periprostatic mesenchyme which intimately contacts UGS epithelium where buds are specified. This was considered the putative TCDD site of action in the UGS for inhibition of ventral and dorsolateral prostatic bud specification. Thus, hyperactivation of AHR signaling appears to disrupt dorsoventral patterning of the UGS, reprogramming where prostatic buds are specified, and prostate lobes are formed. Disrupted axial patterning provides a new paradigm for understanding how in utero TCDD exposure causes ventral prostate agenesis and may shed light on how TCDD impairs development of other organs.

WNT5A selectively inhibits mouse ventral prostate development

The establishment of prostatic budding patterns occurs early in prostate development but mechanisms responsible for this event are poorly understood. We investigated the role of WNT5A in patterning prostatic buds as they emerge from the fetal mouse urogenital sinus (UGS). Wnt5a mRNA was expressed in UGS mesenchyme during budding and was focally up-regulated as buds emerged from the anterior, dorsolateral, and ventral UGS regions. We observed abnormal UGS morphology and prostatic bud patterns in Wnt5a null male fetuses, demonstrated that prostatic bud number was decreased by recombinant mouse WNT5A protein during wild type UGS morphogenesis in vitro, and showed that ventral prostate development was selectively impaired when these WNT5A-treated UGSs were grafted under under kidney capsules of immunodeficient mice and grown for 28 d. Moreover, a WNT5A inhibitory antibody, added to UGS organ culture media, rescued prostatic budding from inhibition by a ventral prostatic bud inhibitor, 2,3,8,7-tetrachlorodibenzo-p-dioxin, and restored ventral prostate morphogenesis when these tissues were grafted under immunodeficient mouse kidney capsules and grown for 28 d. These results suggest that WNT5A participates in prostatic bud patterning by restricting mouse ventral prostate development.